1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an interlaced rate control (IRC) method and apparatus for efficiently controlling reverse traffic.
2. Description of the Related Art
Generally, in a Code Division Multiple Access (CDMA) mobile communication system, multimedia service is supported using the same frequency band. Mobile stations simultaneously transmit data to a base station, and identification of the mobile stations is achieved through spreading codes uniquely assigned to the mobile stations.
Reverse data transmission from a mobile station to a base station is performed over a reverse packet data channel (R-PDCH) by the physical layer packet (PLP), and a packet length is fixed. A data rate is variable for each packet, and a data rate of each packet is controlled depending on a power of a mobile station transmitting the corresponding packet, a total amount of transmission data, and a rate control bit (RCB) provided from a base station over a forward rate control channel (RCCH).
A base station determines reverse rates of mobile stations using Rise over Thermal (RoT), which is a ratio of the total reception power to thermal noises, or a load obtained from signal-to-noise ratios (SNRs) of mobile stations in service. When RoT is used, a reverse rate of a mobile station is controlled so that RoT of the corresponding mobile station approaches a reference RoT, and when RoT is unavailable, a reverse rate of a mobile station is controlled so that a load of the corresponding mobile station approaches a reference load. That is, a base station determines whether to increase, decrease, or hold a data rate of each mobile station based on the RoTs of all mobile stations in service, the total amount of transmission data, and power status. If the rate of a mobile station is efficiently controlled, throughput of the entire system can be increased.
Information for the rate control of a mobile station determined by a base station is transmitted to the corresponding mobile station in the form of a reverse control bit (RCB). If an RCB value received from a base station is ‘+1’ indicating ‘rate up’, a mobile station increases a reverse rate in the next transmission interval. If the RCB value is ‘−1’ indicating ‘rate down’, the mobile station decreases the reverse rate in the next transmission interval. If the RCB value is ‘0’ indicating ‘rate hold’, the mobile station holds the current reverse rate in the next transmission interval.
In certain systems, a base station controls a traffic-to-pilot power ratio (TPR) of a mobile station instead of controlling a data rate of the mobile station. In a conventional mobile communication system, reverse transmission of a mobile station is power-controlled by a base station. In the power control process, a mobile station directly controls power of the pilot channel according to a power control command received from a base station, and controls channels other than the pilot channel depending on the TPR, which has a fixed value. For example, if the TPR is 3 dB, this indicates that a power ratio of a traffic channel to a pilot channel transmitted by a mobile station is 2:1. Therefore, a mobile station determines a power gain of the traffic channel so that the traffic channel should be two times higher than the pilot channel in terms of power.
Even for other types of channels, a gain of the corresponding channel has a fixed value compared with a gain of a pilot channel. In a method of controlling TPR by a base station, in controlling reverse transmissions by a plurality of mobile stations of a base station through scheduling, a system informs TPR allowed for each mobile station instead of directly informing the scheduled result as a data rate. Here, TPR is increased according to an increase in a data rate. For example, if a data rate is increased two times, power assigned to a traffic channel by a mobile station is increased about two times, which means TPR is doubled.
In a conventional mobile communication system, a relationship between a data rate of a reverse traffic channel and TPR is previously known to a mobile station and a base station from an information table. In practice, therefore, controlling a data rate of a mobile station is equivalent to controlling a TPR of a mobile station. Herein, a description will be made of only a method for controlling a data rate of a mobile station by a base station.
FIG. 1 is a flowchart illustrating an operation of determining a reverse rate by a mobile station according to the prior art. The mobile station can support at least 9.6 Kbps, 19.2 Kbps, 38.4 Kbps, 76.8 Kbps, 153.6 Kbps, and 307.2 Kbps for R-PDCH, and increases, decreases, or holds a reverse rate step by step according to a rate control bit (RCB).
Referring to FIG. 1, in step 110, a mobile station receives a rate control bit (RCB) and analyzes the received rate control bit. In step 120, the mobile station determines whether a value of the rate control bit indicates ‘rate up’. If a value of the rate control bit is ‘+1’, indicating ‘rate up’, in step 130, the mobile station sets a rate to be used in the next time interval to a value (or rate) increased one step higher than a rate for the current time interval, and then proceeds to step 170.
However, if a value of the rate control bit is not ‘+1’, indicating ‘rate up’, the mobile station determines in step 140 whether a value of the rate control bit indicates ‘rate down’. If it is determined that a value of the rate control bit is ‘−1’, indicating ‘rate down’, in step 150, the mobile station sets a rate to be used in the next time interval to a value decreased one step lower than a rate for the current time interval, and then proceeds to step 170.
However, if it is determined that a value of the rate control bit is not ‘−1’, indicating ‘rate down’, in step 160, the mobile station sets a rate to be used in the next time interval to the same value as a rate for the current time interval. In step 170, the mobile station transmits a data frame in the next time interval according to the determined rate.
FIG. 2 is a timing diagram illustrating an operation of determining a reverse rate by a mobile station according to the prior art. The RCB is transmitted one time from a base station to a mobile station for each transmission interval. The RCB is used to control a reverse rate of R-PDCH for the next transmission interval of a mobile station.
Referring to FIG. 2, in a time interval t0, a mobile station transmits a data frame over a packet data channel (PDCH) at a rate of 9.6 Kbps (see 210). In the time interval t1, a base station determines whether to increase, decrease, or hold a data rate of a mobile station in consideration of an RoT, a buffer status, and a power status of the corresponding mobile station, generates an RCB according to the determination result, and transmits the generated RCB to the mobile station (see 220). Then the mobile station receives the RCB, analyzes the RCB, and determines whether to increase, decrease, or hold a rate of PDCH in the next time interval t2.
However, in such a rate control method, due to a delay between a time where RCB is generated in a base station and a time where the RCB is actually applied in a mobile station, a base station cannot efficiently perform rate control on its mobile stations.
For example, in a time interval t5, a base station receives a data frame from a mobile station at a rate of 153.6 Kbps, and in the same time interval, the base station determines to increase a data rate of the mobile station by one step from the current rate of 153.6 Kbps according to conditions of other mobile stations, generates a corresponding RCB(+), and transmits the generated RCB(+) to the mobile station. Actually, however, because the RCB(+) is transmitted for a time interval t6, a time interval where the RCB(+) is actually applied becomes t7 taking into account a time required when the mobile station receives the RCB(+) and analyzes the RCB(+). As a result, in the time interval t7, the mobile station sets a rate 614.4 Kbps, which is increased one step higher than a rate 307.2 Kbps for the previous time interval t6.
When several mobile stations simultaneously transmit reverse data, data transmitted by other mobile stations acts as interference to a signal of a particular mobile station. Therefore, a base station performs a control operation in such a manner that all rates or all RoT values of data transmitted by mobile stations in the cell should not exceed a particular threshold. In this case, when the base station increases a data rate of a particular mobile station, the base station must decrease data rates of the other base stations. Accordingly, data throughputs of mobile stations receiving a data service from a particular base station depend upon the efficiency of the reverse rate control.
However, as illustrated in FIG. 2, a mobile station determines whether to increase, decrease, or hold a next data rate in comparison with a data rate used in the previous time interval, depending on an RCB received from a base station. In this case, due to a delay between a time when the RCB is generated in a base station and a time when the RCB is actually applied in a mobile station, reverse rate control cannot be efficiently performed, leading to deterioration in data throughput of the entire system.